1. Field of the Invention
This invention relates to a shielded cable used for electrically connecting an electrical equipment in a vehicle such as an automobile.
2. Description of the Related Art
In a vehicle such as an automobile, shielded cables have been used for electrical connection to various electrical equipments. FIG. 1 is a cross-sectional view showing a construction of a shielded cable according to a related art.
In FIG. 1, reference numeral 10 denotes the shielded cable comprising a plurality of signal wires 11 and a drain wire 12, which are twisted together, a shielding layer 13 covering an outer periphery of these wires 11, 12, and an insulating sheath 14 covering an outer periphery of this shielding layer 13. The signal wire 11 comprises a conductor 11a and an insulating covering member 11b covering an outer periphery of this conductor 11a. 
In this construction, external noises are intercepted by the shielding layer 13 and the intercepted noises are fed to an earth through the drain wire 12. Good signals are supplied to various electrical equipments through the signal wires 11.
With respect to the commonly-used specification of the shielded cable 10 of this kind according to the related art, the cross-sectional area of the signal wire 11 is 0.3 mm2, the outer diameter of the conductor 11a of the signal wire 11 is 1.4 mmxcfx86, the material of the insulating covering member 11b is polyvinyl chloride (PVC), the material of the shielding layer 13 is copper or aluminum, and the material of the insulating sheath 14 is polyvinyl chloride, polyethylene or the like.
On the other hand, recently, with the rapid development of a car navigation system and a DVD equipment, a monitor, a DVD unit and so on have now been mounted on automobiles. However, since an image, a voice and so on of these electrical/electronic equipments have become finer and higher in quality, for example, with respect to the monitor of the car navigation system, there has occurred a phenomenon in which blurring develops in an enlarged image of a road map or the like, and it has been desired to take any countermeasure.
And besides, in the shielded cable according to the related art, the signal wires 11 and the drain wire 12 are formed into the twisted-wire combination structure, and therefore it is not easy to process an end portion of the wires 11, 12 (for connection to connector terminals), and much time is required for the processing of the end portion thereof. In addition, since the insulating sheath 14 and the shielding layer 13 need to be removed over a length of about 80 mm to provide a length of processed end portion, there is encountered a problem that the shielding performance is degraded.
Furthermore, in connection with the standards of connectors of electrical/electronic equipments to be newly mounted on automobiles, the outer diameter of the signal wire 11 is required to be 1.3 mmxcfx86, and with respect to the standards of the conventional connectors, the outer diameter of the signal wires of 1.4 to 1.6 mmxcfx86 prevails. Therefore, the outer diameter of the signal wires must be narrowed; otherwise such outer diameter would not be suited for the connectors of the equipments to be newly mounted.
This invention seeks to solve these problems of the related art, and an object of the invention is to provide a shielded cable which maintains a shielding performance, can be easily subjected to an end processing, and can be sufficiently adapted for the latest electronic/electrical equipments to be newly mounted on a vehicle such as an automobile.
The inventor of the present invention has dedicated to a study to solve the above problems, has obtained the following findings, and has completed the present invention.
Troubles encountered with the electronic/electric equipments newly mounted on an automobile are much concerned with the characteristic impedance of the signal wires and its value of the signal wires according to the related art is about 50xcexa9. It is thought that when the shielded cable according to the related art is used and connected to an equipment, such as a DVD unit, a monitor, and the like, reflection occurs between the signal-supplying side and the equipment because of unmatching of the impedance, so that the above troubles occur. Therefore, it has been confirmed that by increasing the characteristic impedance of the signal wire to about 75xcexa9 (xc2x110%), the matching can be obtained so that the above troubles can be overcome. On the other hand, in connection with the standards of the connectors of the electronic/electrical equipments to be newly mounted, the outer diameter of the signal wire need to be 1.3 mmxcfx86. It has been confirmed that in view of these, by adjusting the size of a cross-sectional area of the conductor of the signal wire and the dielectric constant of the insulating covering member of the signal wire, the required characteristic impedance can be obtained. Incidentally, the dielectric constant of polyvinyl chloride used to form the insulating covering member of the signal wire according to the related art is about 5.0.
In order to solve the above problems, according to a first aspect of the invention, there is provided a flat-type shielded cable comprising: a plurality of signal wires having insulating covering members and conductors, respectively; and a drain wire, wherein the plurality of signal wires ad the drain wire are juxtaposed to one another in closely-contacted relation to one another; a shielding layer and an insulating sheath cover an outer periphery of the juxtaposed wires sequentially; an outer diameter of the signal wire is in a range of 1.25 to 1.40 mm; a cross-sectional area of the conductor of the signal wire is in a range of 0.03 to 0.09 mm2; the insulating covering member of the signal wire is made of an insulative material having a dielectric constant of 1.7 to 2.8; and a characteristic impedance of the signal wire is 75xcexa9 (xc2x110%)
According to a second aspect of the invention, there is provided the shielded cable according to the first aspect of the invention, wherein the insulating covering member of the signal wire is foamed polyethylene.
According to a third aspect of the invention, there is provided the shielded cable according to any one of the first and second aspects of the invention, wherein a cross-sectional area of the drain wire is in a range of 0.20 to 0.57 mm2.
According to a fourth aspect of the invention, there is provided the shielded cable according to any one of the first to third aspect of the invention, wherein the drain wire is provided at one of ends of the plurality of juxtaposed signal wires having the insulating cover member, respectively.
According to a fifth aspect of the invention, there is provided the shielded cable according to any one of the first to third aspect of the invention, wherein the drain wire is provided among the plurality of juxtaposed signal wires having the insulating covering members, respectively.